Effects that change in strain ratio causes to structural fire resistance of a steel member - formulation of stress-strain relationships considering effects of strain ratio and verification based on in-plane analyses

摘要

In this paper, effects that change in strain ratio for steel at elevated temperature causes to fire resistance of a steel member are discussed. Stress and strain relationships from ambient temperature to 800℃ for JIS SN400B are obtained by conducing coupon tests under various strain ratio values. The stress and strain relationships are formulated to be agreed with the coupon test results. For the above formulated relationships, a negative gradient of the stress observed at the slow strain ratio coupon test is considered. By using an in-plane numerical analysis using a non-liner beam element developed in this study, behavior and load bearing capacity of a steel cantilever beam under condition of constant load and increasing member temperature are investigated. From investigation of the numerical results, it was found that the strain ratio at the elevated temperature essentially had the same effect as heating rate [℃/min] of the steel beam. That is, in case of the rapid heat rate, which is exemplified by the case of the unprotected steel member subjected to post-flashover fire, the beam possesses the large load bearing capacity in comparison with the beam with the slow heat rate.